Fluidized beds are used in a number of applications for the heating and cooling of materials. They exhibit good heat transfer characteristics and a uniform bed temperature profile, and have the capability of being quickly turned on because they are able to store heat or refrigeration. They are usually designed to utilize heat exchange panels or coils when the heating or cooling loadsignificantly exceeds the capability of the fluidizing gas flow to provide it.
A fluidized bed, in its simplest form, is a bed of particulate material supported on a distribution plate through which gas or liquid is forced at a velocity sufficient to cause the particles to separate and to act as a fluid. The primary physical characteristics affecting a fluidized bed are particle size and distribution, particle density, fluid viscosity, and fluid density. Further, the viscosity of fluidizing gases drops off rapidly with a reduction in temperature, which necessitates an increase in fluidizing flow.
One technique used for fluidizing a bed is known as pulsing, which is the cycling of the fluidizing flow of gas or liquid through "on" and "off" phases. Pulsing is found to reduce the quantity of gas or liquid required to maintain sufficient fludization in the bed to carry out the intended heat treatment or cryogenic process. Unfortunately, the present practice of on/off pulsing does not provide for a uniform flow through the heat transfer device, if one is used to heat or cool the bed, or through the bed itself. This lack of uniform flow leads to slugging in the heat exchanger and/or a failure to maintain a constant temperature and pressur in the bed, which, in turn, leads to an inability to meet process specifications and inefficiency in the utilization of the very gas or liquid, which the pulsing technique is attempting to save over its predecessor, the continuous flow.
It is clear, then, that some other arrangement for utilizing the fluidizing medium is necessary if slugging is to be avoided and constant temperatures and pressures are to be achieved without a loss in gas utilization efficiency.